Bill's large missile scratchbuild thread

[Bill S]

I'm working on my first real mid-power rocket (excepting an Super Big Bertha and Executioner). It uses LOC 3" tubing, and should be 37.5" long. Image of final version:



First up, the nosecone. The plastic was awful, with lots of deep grooves and marks. I sanded it with 220grit, then 320, then primed it with some Rustoleum filler primer. I then puttied the heck out of it with Bondo Glazing putty, followed by lots of sanding. After several rounds of prime, chase and fix the defects, etc, I ended up with this primed up nosecone:



I decided to work on the rear fins (1/8" plywood) next. I cut them out of the plywood square with both a hacksaw (32 tooth blade) and an Xacto hobby saw. Results were fair; I wish I had better equipment and a place to set up some power tools, etc, but alas I don't. There was a LOT of sanding to try and make them as close as possible in size. I gave the fins a quick rounding on the edges.

A sample fin with through the wall tab on the bottom:


Next up was one of the large "fins" on the side of the rocket. I used 1/2" x 1" x 15 1/2" long balsa. It turned out to be denser than expected, unfortunately.



I then put in a slot in the back of the "side fin" assembly so that the rear fin could go through it. This was a little tricky - I had to drill a hole at the end of the slot where I wanted it to stop, then marked an 1/8" wide slot to the back of the "side fin". I used a metal ruler taped in place as a stop so the Xacto saw wouldn't go too far forward by accident.



View of the slot after cutting, but before I removed the wood:


The side fin with the slot opened up. I need to keep the left over piece because after the fins are glued in place and the side fins are put in place over them, there will be a nasty gap in the rear of the side fin where it meets the rear of the body tube. I'll have to cut off a bit of the angled portion to glue in place so you can't tell it was removed.


Test fit with a rear fin in place:


More to come soon.

 

[neil_w]

Wow I wouldn't have thought to cut a slot out of the balsa like that. When I made fins assemblies sort of like that on a recent build, I built them up with a 3-piece sandwich, and just left the necessary gap in the middle piece for the inserts.

But you seem to have gotten pretty much perfect results with the saw. Well done.

 

[Bill S]

Thanks Neil. Looking at the fin assembly you did, the logical thing would have been to do it the way you did it. Cutting out properly sized slots out of a solid block would have been challenging.

 

[Hobie1dog]

Outstanding, thanks for all the pictures. Looking forward to seeing it finished.

 

[crossfire]

Very nice work.

 

[Bill S]

Today's progress:
Cut out 4 strakes for the front of the rocket. They go in front of the rear intake assemblies. Also cut out 4 secondary fins that go on top of the rear intake assemblies.


Here are the strake, rear intake assembly, top fin and rear fin show as they will go on the rocket:


All 4 assemblies awaiting attachment to the rocket. I haven't cut the slots in the rear for the rear fin on 3 of the 4 assemblies yet (I made an error on the first one, which I have to correct, and don't want to make the same mistake on the other 3).


Next up, cut the body tube to length, putty up the spirals, sand, etc, in preparation for adding the fin lines and cutting fin slots, etc.

 

[Bill S]

Yesterday, I cut out an centering ring (3" to BT-60) out of some heavy "cardboard" I had lying around. Its thicker and heavier than I would like but its what I could do.



Built the stuffer tube, and attached it to the motor mount. Annoyingly when I was epoxying the front centering ring in position (checked alignment with a square so I know it was correct), the ring slipped down a bit while the epoxy was setting up. I had it standing vertically so the epoxy wouldn't drip all over the place. Mistake... Next time I'll just put a tiny bit of epoxy around the tube to secure it and let it lay on its side so the ring can't move. Fortunately the ring didn't move so much as to render the assembly unable to be inserted into the body tube. I also test fitted the styrene tube that will contain the kevlar cord that goes from the rear of the motor mount tube to the front of the rocket, serving as the "shockcord".

 

[neil_w]

Annoyingly when I was epoxying the front centering ring in position (checked alignment with a square so I know it was correct), the ring slipped down a bit while the epoxy was setting up. I had it standing vertically so the epoxy wouldn't drip all over the place. Mistake... Next time I'll just put a tiny bit of epoxy around the tube to secure it and let it lay on its side so the ring can't move.

This is an application for which I would stick with wood glue. It'll tack fairly quickly, won't drift while drying, and will be plenty strong, especially after filleting. And for initial gluing, you can do it horizontally; just reposition vertically for filleting after the ring is securely in place.

If you're set on using epoxy, then you could use the common HPR builder's technique: tack the ring on with CA and then fillet with epoxy (but again, I'm not sure why bother on a paper and wood rocket).

Finally: to square up centering rings (surprisingly challenging!) I've taken to using a "pusher": I wrap some cardstock around the body tube, square up the ends, and then use it to push the ring into position. Has worked very well for me so far.

 

[jmasterj]

What made you decide to use a stuffer tube separate from the motor mount, rather than just a longer motor mount? Parts availability?

 

[Bat-mite]

Youi clearly have patience and a steady hand ... two things I lack!

 

[Bill S]

Neil, normally I would have used wood glue to attach the front centering ring, but I was concerned that the glue would bite and get stuck before I could align the ring with the square (I checked 4 places to confirm that I had it straight). I'm not a huge fan of epoxy, but there are times where it is handy. I suspect I was wrong about the wood glue though. I plan to use wood glue to attach the fin tabs and rear ring for sure.

I hadn't thought of using a pusher to align the ring. Hmmm...

jmasterj, mainly I didn't want to risk roasting the stuffer tube if I used BT-55. The motor mount is an 29mm. BT-60 gives plenty of room I think. Now I did cut down the size of the motor tube to 5", the longest I thought I'd need (rocket should work okay on F67 all the way up to G-80, no need for an longer tube.

 

[neil_w]

Neil, normally I would have used wood glue to attach the front centering ring, but I was concerned that the glue would bite and get stuck before I could align the ring with the square (I checked 4 places to confirm that I had it straight). I'm not a huge fan of epoxy, but there are times where it is handy. I suspect I was wrong about the wood glue though. I plan to use wood glue to attach the fin tabs and rear ring for sure.

Grabbing is a concern; now that I think about it. It depends to some extent on how tight the rings are. I like them them fairly tight, and in that case using white glue for the initial bond might be better, and then wood glue for the fillets.

I hadn't thought of using a pusher to align the ring. Hmmm...

The pusher technique works great, but (as follow-up to the previous comments) might be troublesome if using wood glue on tight rings for the initial bond, since it can require significant force to push the ring into place.

In TVM's recent video on installing couplers, he talks about using more glue to provide extra lubrication, and that might apply here as well.

 

[Bill S]

More progress...epoxying in the hollow styrene tube that the kevlar runs through from the back of the motor mount to the front of the rocket:



Cutting the fin slots. Used a large 3" coupler inside the tube to help it keep its shape while being cut (freehand with an Xacto knife, which worked out better than expected).



Test fitting the rear fins with the motor mount in place:



Test fitting the long intake fin and forward strake:



Wrapping the styrene kevlar tube with some aluminum duct tape to give it some heat protection (has worked well so far with 2 other rockets):




Test fitting the fins in place with a fin template to confirm everything was in alignment. Only goof I made was putting the end of the motor mount flush with the end of the body tube - I had designed it with it extending .25" beyond, but as I've glued up the motor mount forward centering ring I can't fix it now. It has a negligible effect on stability anways.



Closer look at the interior. Fins are not yet glued in - I just epoxied in the motor mount and one fin to keep everything in alignment while it dries. I should have pictures of that tomorrow I hope.

 

[jmasterj]

Does that motor mount extension change affect your retention choice or just the relationship with the bottom of the airframe ?

 

[Bill S]

Does that motor mount extension change affect your retention choice or just the relationship with the bottom of the airframe ?

Fortunately just the relationship with the bottom of the body tube. I have 5/16" of the motor tube sticking out, and the motor retention (Aeropack 29L) requires 1/4" so its good.

 

[Bill S]

Got the fin tabs filleted up, and put the rail buttons on for a test - I need to remove them and epoxy the backings into the tube.








I cut the slots in the rear of the intake fins/strakes, and attached the 1/4" dowels to the intake fronts.



I found some 1/4" bungee type material that I am going to try to attach the shockcord (#200 kevlar) to the nosecone. Drilled 2 1/4" holes, fed through and put loops on each end, and fed the kevlar through those.

 

[Bill S]

Sorry for the delay in updates. The weather wasn't good to work outside, and my wife was injured so I was spending more time caring for her (will be for several weeks more I think). Nonetheless, I was able to attach the fin/intakes/strakes and putty everything up. Sanding the putty is progressing slowly, but its getting there. Hopefully over the weekend I will be able to put on some primer, then fix the newly visible defects, etc.

Pic:

 

[BABAR]

Hope your wife isn’t in too much pain and completely recovers quickly!

 

[Bill S]

Hope your wife isn’t in too much pain and completely recovers quickly!

She broke her elbow, and just had surgery today. Its going to take a while. Thanks for the concern.

 

[Bill S]

Been a while since I updated this. The rocket is ready to fly, though I have not yet come up with any decals for it. A lack of inspiration and I have to get off my butt and do something about that. I've been looking at various air to air missiles, and by and large they don't have a lot of markings, though some colored bands around the body tube and some random fittings, etc, might do the trick.

23.5oz ready to fly, 37.75" long. It should do well on F44-4s, F67-6, G74-6, maybe an G80 if visit MDRA. I'm using an chute protector for the first time, and once I figure out how to use my new Jolly Logic chute release it'll go on there (I'm practicing on it with my old Estes Vapor which is nearing retirement so if I mess up and crash it I won't cry about it).

 

[jmasterj]

If you want any chute release help, I'd be happy to show you how I use mine. I've used it a lot this year. I won't be at this month's NARHAMS launch, but maybe next month.

 

[Pepe Le Pew]

Bill, great looking rocket! I hope your wife feels better soon. Joser

 

[Bill S]

Finally finished the rocket; hope to fly it next weekend.

I discovered while searching actual missiles for decal ideas, that may have subconciously imitated an actual surface to air missile (RIM-67 Standard). In reality I borrowed the fin elements from the Estes QCC Explorer, and modified from there. The decals were definitely inspired by actual missiles, which in general seem to be light on detailing.